Bolt looseness diagnosis for steel beam connection using wireless
impedance-based method
Chẩn đoán tình trạng lỏng bu-lông cho liên kết dầm thép bằng phương pháp
đo trở kháng không dây
Thanh Canh Huynha,b*,The Duong Nguyena,b, Quang Nhat Phama,b
Huỳnh Thanh Cảnha,b*, Nguyễn Thế Dươnga,b, Phạm Quang Nhậta,b
aCenter for Construction, Mechanics and Materials, Institute of Research & Development, Duy Tan University,
Da Nang, 550000, Vietnam
aTrung tâm Xây dựng, Cơ học và Vật Liệu, Vi
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Tóm tắt tài liệu Bolt looseness diagnosis for steel beam connection using wireless impedance-Based method, để xem tài liệu hoàn chỉnh bạn click vào nút DOWNLOAD ở trên
iện Nghiên cứu và Phát triển Công nghệ Cao, Đại học Duy Tân,
Đà Nẵng, Việt Nam
bFaculty of Civil Engineering, Duy Tan University, Da Nang, 550000, Vietnam
bKhoa Xây dựng, Đại học Duy Tân, Đà Nẵng, Việt Nam
(Ngày nhận bài: 08/11/2019, ngày phản biện xong: 14/11/2019, ngày chấp nhận đăng: 4/5/2020)
Abstract
Bolted connections are widely used to link load-carrying members in steel structures. The bolt fastening force is
critically important to guarantee the strength of a bolted joint. Bolt looseness could carry potentials that may result in
the failure of the bolted joint, threatening the stability of a whole structural system. This study has been motivated to
monitor the bolt looseness in a steel beam connection by using a wireless impedance-based method. The main idea of
the method is to monitor bolt looseness-induced changes in electromechanical impedance responses of the joint.
Wireless impedance sensing technology was applied to perform automated impedance measurements with minimal
costs. Also, the piezoelectric interface technique was implemented to predetermine sensitive frequency ranges in the
impedance measurements. The feasibility of the method was evaluated for impedance monitoring of a realistic steel
beam joint with multiple bolts. The obtained results showed that loosened bolts in the tested joint were successfully
detected, thus demonstrating the feasibility of the method and showing its promising future applications to steel
structures in the field.
Keywords: Bolt looseness detection; bolted connection; steel structures; wireless impedance sensor; impedance
response; PZT interface.
Tóm tắt
Mối nối bu-lông được sử dụng rộng rãi để liên kết các thành phần chịu lực trong kết cấu thép. Trong đó, lực siết của bu-
lông đóng vai trò rất quan trọng trong việc đảm bảo độ cứng của liên kết. Việc mất mát lực siết mang những nguy cơ
dẫn đến sự phá hoại liên kết, sau đó là đe dọa đến sự ổn định của toàn bộ hệ thống kết cấu. Bài báo này giới thiệu một
phương pháp giám sát việc lỏng bu-lông cho liên kết dầm thép bằng kĩ thuật đo trở kháng không dây. Ý tưởng chính
của phương pháp này là giám sát sự biến đổi của tín hiệu trở kháng của liên kết trước và sau khi bu-lông bị lỏng.
*Corresponding Author: Thanh Canh Huynh; Institute of Research & Development, Duy Tan University, 550000,
Vietnam; Faculty of Civil Engineering, Duy Tan University, Danang, 550000, Vietnam.
Email: huynhthanhcanh@duytan.edu.vn
02(39) (2020) 30-36
T.C.Huynh, T.D.Nguyen, Q.N.Phạm / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(39) (2020) 3 31930- 6
Công nghệ cảm biến không dây được áp dụng để thực hiện các phép đo trở kháng một cách tự động từ xa với chi phí
thấp. Ngoài ra, để xác định trước các dải tần số nhạy trong các phép đo trở kháng, kỹ thuật giao diện áp điện (PZT
interface technique) được ứng dụng. Tính khả thi của phương pháp trên được đánh giá thông qua các đo đạc trở kháng
trên một liên kết dầm thép thực với nhiều bu-lông. Kết quả thu được cho thấy các bu-lông bị nới lỏng trong liên kết dầm
thép được chẩn đoán một cách chính xác, qua đó cho thấy tính khả thi và triển vọng ứng dụng của phương pháp đo trở
kháng không dây vào các công trình thực tế.
Từ khóa: Cảm biến không dây; điện-cơ; giao diện áp điện; cảm biến áp điện; ứng xử trở kháng; theo dõi sức khỏe công
trình.
1. Introduction
Bolted connections have been widely used in
steel structures such as bridges, pipelines, and
buildings. The strength of a bolt connection is
guaranteed by the preload of bolts. However, as
discontinuous parts of structures, bolted
connections are often influenced by severe
repeated loading and various environmental
conditions. As the result, bolt looseness can be
occurred in the connection, carrying potentials
that could threaten the stability of a whole
structure.
Recently, the impedance-based technique
has been studied by many researchers and
emerged as an innovative damage detection
tool [1-5]. Several research attempts have been
made on employing the impedance-based
technique to damage detection in bolted
structures [3,6,7]. Basically, the impedance-
based method utilizes high-frequency
electromechanical (EM) impedance responses
measured by a piezoelectric (e.g., PZT) sensor
to assess the integrity of a structure. The
utilization of high frequencies allows the
technique to detect minor changes in the
structure induced by damage events.
To efficiently monitor in-situ structures, the
impedance-based technique has been integrated
with low-cost wireless impedance sensors [8].
The wireless impedance sensing technology
would allow to perform automated impedance
measurements with minimal costs. However,
the application of the wireless impedance-based
technique has been limited by the narrow
frequency range of the low-cost wireless
impedance sensor, currently less than 100 kHz
[8]. To obtain accurate results, therefore, it is
important to predetermine the sensitive
frequency bands below 100 kHz for wireless
impedance measurements in practices.
The piezoelectric interface technique (i.e.,
PZT interface) has been recently developed and
can be a promising solution to deal with the
above issue [3-5]. The technique is basically an
alternative attachment method for the PZT
sensor. The PZT is indirectly attached to the
host structure via a substrate structure called
‘interface’. The structural and geometrical
properties of the PZT interface can be adjusted
to create strong resonances (i.e., sensitive
frequencies) in a desired frequency band,
typically below 100 kHz to enable wireless
impedance measurements.
This study introduces a wireless impedance-
based method, that combines the wireless
impedance sensing technology with the
piezoelectric interface technique, for
monitoring the bolt-loosening occurrence in a
steel beam connection. The main idea of the
method is to monitor bolt looseness-induced
changes in EM impedance responses of the
joint. The feasibility of the method was
evaluated for bolt-loosening monitoring of a
realistic steel beam joint. The obtained results
showed that loosened bolts in the test structure
were successfully detected, thus demonstrating
the feasibility of the method and showing its
promising future applications to steel structures
in the field.
T.C.Huynh, T.D.Nguyen, Q.N.Phạm / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(39) (2020) 30-332 6
2. Wireless impedance-based method
2.1. Wireless impedance sensor node
The research group at Pukyong National
University has developed smart wireless
impedance sensor nodes [3,9]. In this paper, a
low-cost wireless impedance sensor, SSeL-I,
developed by Nguyen et al. [9] was adopted to
acquire the impedance responses from bolted
connections. Figure 1 shows a prototype and
schematic design of the wireless impedance
sensor SSeL-I. The wireless sensor node has
three layers: a battery board, the Imote2 sensor
platform, and the SSeL-I impedance sensor
board. The Imote2 platform is used to control
the impedance board and the battery board is
used to power the sensor node.
The key component of the impedance board
is a low-cost impedance chip, AD5933 that
enables the high-frequency impedance
measurement. The AD5933 chip has the
following embedded multi-functional circuits:
function generator, digital-to-analog converter,
current-to-voltage amplifier, anti-aliasing filter,
ADC, and discrete Fourier transform (DFT)
analyzer. The AD5933 interacts with an
ADG706 multiplexer to allow sixteen sensing
channels on a single sensor node. An SHT11
sensor is also integrated into SSeL-I board to
monitor the temperature and humidity of the
environment. The Imote2 platform is designed
with a PXA27x processor which has a clock
speed of 13-416 MHz, SRAM of 256 kB, flash
memory of 32 MB and SDRAM of 32 MB.
This platform also integrates with many I/O
options and a wireless radio CC2420 (2.4 GHz
Zigbee RF) for data transmission. The large
memory and high operating speed of Imote2
allow it to enable advanced and complicated
SHM techniques. Although the AD5933 chip
allows the scanning frequency below 100 kHz,
a SSeL-I unit costs only 300 USD, which is
much lower than the prices of commercial
impedance analysers.
Figure 1. A prototype and schematic design of SSeL-I impedance sensor
An operating software, the so-called ‘SSeL
SHM Tools’, was developed for the SSeL-I
sensor node. The tool is programmed on
TinyOS platform which is a free and open
source with a huge library for designing
wireless sensor networks. The ‘SSeL SHM
Tool’ embedded software was described in
details in Nguyen et al. [9]. At the base station,
the user makes a sensing request to the remote
node from the captain node by using the
‘RemoteControl’ component of the tool. The
request should contain the measuring frequency
range and the number of channels defined for
the remote node. When the request is received,
the remote node starts to measure the
impedance responses in the defined frequency
range via the defined channels. When the
measurement is completed, data is wirelessly
transmitted to the captain node. At the base
station, the measured impedance signals are
calibrated. Then, the impedance features are
extracted by using the ‘ImpedanceComponent’
of the tool for the condition assessment of the
monitored structure.
T.C.Huynh, T.D.Nguyen, Q.N.Phạm / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(39) (2020) -3 33 30 6
2.2. Piezoelectric interface technique
The piezoelectric interface technique is a
solution to predetermine effective frequency
bands of impedance responses and to reduce
them to below 100 kHz for the wireless
impedance sensing. Huynh et al. [4] developed
a PZT interface which is a portable device for
wireless impedance monitoring of prestressed
systems. In this study, the PZT interface was
employed to monitor the structural condition of
bolted structures. The PZT interface is surface-
attached to a bolted connection to sense the
impedance responses which represents the
coupling between the interface structure and the
connection. When the structural parameters of
the bolt connection are altered by damages
(e.g., bolt looseness), the coupling responses
will be changed. Consequently, the impedance
signatures of the connection can be shifted.
Thus, the structural integrity of the bolting
structure can be estimated by monitoring the
impedance changes obtained from the PZT
interface.
miPZT
Kms
ks
cs
Interface Host StructurePZT Sensor
Z (ω)sZ (ω)i
K
i
ci
k
Structure
PZT
V()
I()
( )F
( , , )k c m
PZT M K
C
Coupled Electro-Mechanical Admitance
Y=Re(Y)+jIm(Y)
sin( )I i t
sin( )V v t
Coupling Interaction
1-dof Electromechanical Impedance
Model (Liang et al. 1996)
Structure
PZT
V()
I()
( )F
( , , )k c m
PZT M K
C
Coupled Electro-Mechanical Admitance
Y=Re(Y)+jIm(Y)
sin( )I i t
sin( )V v t
Coupling Interaction
1-dof Electromechanical Impedance
Model (Liang et al. 1996)
Figure 2. PZT interface -host structure system
Figure 2 shows a simplified impedance
model that theoretically describes coupled
dynamic responses of a PZT interface-host
structure system [5]. The PZT interface-host
structure is modeled as a 2-degree of freedom
(dof) spring-mass-damper system, in which mi,
ci, ki and ms, cs, ks are the masses, damping
coefficients, and spring stiffness of the interface
and the host structure generated at the PZT
driving point. In the model, one dof refers to
the host structure (i.e., a bolted joint)
represented by the impedance Zs and the other
refers to the interface represented by its
impedance Zi.
The resultant impedance of the interface-
bolted joint system at the PZT driving point is
defined as the ratio between the excitation force
Fi and the velocity xi, as follows:
2
11 22 12
22
( )
( ) i
i
F K K K
Z
x i K
(1)
in which, the terms [Kij], i,j = 1, 2 are the
dynamic stiffness of the 2-dof system, as
follows:
2
11 12
2
12 22 ( ) ( )
i i i i i
i i s i s i s
K K m i c k i c k
K K i c k m i c c k k
(2)
The overall impedance of the PZT interface-
bolted joint system can be obtained [1,5], as
follows:
1
2
33 31 11
1 ˆˆ( )
( ) ( ) 1
T Ea a
a a
w l
Z i d Y
t Z Z
(3)
where ˆ (1 )E Exx xxY i Y is the complex Young’s
modulus of the PZT patch at zero electric field;
33 33
ˆ (1 )T Ti is the complex dielectric
constant at zero stress; d31 is the piezoelectric
coupling constant in 1-direction at zero stress;
and wa, la, and ta are the width, length, and
thickness of the PZT patch, respectively. The
parameters and are structural damping loss
factor and dielectric loss factor of piezoelectric
material, respectively.
The 2-dof impedance model should contain
two resonant peaks in its impedance signatures
that represent the two coupled vibration modes
of the PZT interface-host structure system.
When the bolted joint is damaged (e.g., bolt
looseness), its structural parameters (ms, cs, ks)
are altered, resulting in the variation in the
overall impedance according to Eq. (2) and Eq.
(3). For damage detection, the RMSD (i.e., root-
mean-square deviation) index was extracted
Z
T.C.Huynh, T.D.Nguyen, Q.N.Phạm / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(39) (2020) 30-334 6
from the measured impedance signals. The
RMSD index can be computed, as follows [10]:
2 2*
1 1
( ) ( ) ( )
N N
i i i
i i
RMSD Z Z Z
(4)
where iZ and
*
iZ signify the
impedance responses at the ith frequency before
and after a damage event, respectively; N
denotes the number of swept frequencies.
Ideally, RMSD 0 indicates that the bolted
joint is healthy (i.e., no bolt-loosening) and
RMSD > 0 reveals that the joint is loosened
(i.e., bolt-loosening).
3. Experimental evaluation
3.1. Test-setup of steel beam
Figure 3 shows the test-setup of a lab-scaled
steel beam. The beam was assembled from two
single H-shaped beams (H – 200x180x8x100
mm) by splice plates (200x310x10 mm) and 8
bolts at two flanges (d – 20 mm). A PZT
interface having a flexible section (33x30x4
mm) and two outside bonded sections (33x35x5
mm) was designed and surface-mounted at the
middle of the splice plate. The interface was
equipped with a PZT-5A (15x15x0.51 mm) at
the flexible section. The PZT interface was
designed with sensitive frequency bands below
100 kHz.
The SSeL-I impedance measurement system
consists of a laptop connected to a wireless
captain node and a remote SSeL-I node
connected to the PZT interface, see Fig. 3. To
acquire the EM impedance from the bolted
connection, the PZT sensor was excited by a
harmonic voltage of 1 V in the frequency band
of 10-55 kHz by using the remote node. The
acquired impedance signal was then wirelessly
sent back to the laptop via the captain node.
As the healthy state, all bolted were fastened
by the torque of 160 Nm. Four of eight bolts
(i.e., Bolts 1-4) on the splice plate were selected
to introduce four bolt-loosening events. For
each bolt-loosening event, the torque was
loosened to 0 Nm (completely loosened). The
impedance signals were measured before and
after each bolt looseness event.
Figure 3. Test-setup of a steel beam connection
3.2. Wireless bolt looseness detection results
Figure 4 shows the impedance responses of
the bolt connection in the frequency band 10-55
kHz under the loosened case of Bolt 1. Two
resonant bands (i.e., 10-20 kHz and 24-34 kHz)
which are below 100 kHz were observed in the
figure. It is shown that the impedance signature
was sensitively varied according to the loss of
Laptop
Captain
Node
Remote
Node
Bolt 1
PZT Interface
Bolt 2
Bolt 3
Bolt 4
T.C.Huynh, T.D.Nguyen, Q.N.Phạm / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(39) (2020) 3 35 30- 6
bolt torque. The first resonant band in 10-20
kHz experienced both the frequency and
magnitudes shifts while the second one in 24-
34 kHz showed only slight changes in the peak
frequency and almost no noticeable changes in
the magnitudes.
Figure 4. Impedance responses of bolted connection under Bolt 1 Looseness
Figure 5. Wireless bolt-loosening detection results
Figure 5 shows the wireless bolt-loosening
monitoring results using the RMSD index. The
whole frequency range 10-55 kHz was used for
the calculation. It is observed that when the bolt
torque was reduced from 160 Nm to 0 Nm
(completely loosened), the RMSD index was
considerably increased from unnoticeable
values to significant values. Interestingly, when
Bolt 3 was loosened completely, the RMSD
index indicated the highest value among four
bolts (Bolts 1-4). This may be due to the bolted
connection tested in this study was not
completely symmetric. Despite that, these
results demonstrated the feasibility of the
wireless impedance-based method for bolt-
loosening detection in bolted joints.
10 15 20 25 30 35 40 45 50 55
0
100
200
300
400
500
600
Frequency (kHz)
R
ea
l I
m
pe
da
nc
e
(O
hm
)
Torque: 160 Nm
Torque: 0 Nm
10 12 14 16 18 20
50
100
150
200
250
300
350
Frequency (kHz)
R
ea
l I
m
pe
da
nc
e
(O
hm
)
Torque: 160 Nm
Torque: 0 Nm
24 26 28 30 32 34
0
100
200
300
400
500
600
Frequency (kHz)
R
ea
l I
m
pe
da
nc
e
(O
hm
)
Torque: 160 Nm
Torque: 0 Nm
10-55 kHz
10-20 kHz
24-34 kHz
160 Nm 0 Nm
0
10
20
30
40
50
60
70
80
R
M
S
D
(%
)
Torque in Bolt 3
2.18
50.91
160 Nm 0 Nm
0
10
20
30
40
50
60
70
80
R
M
S
D
(%
)
Torque in Bolt 2
1.23
23.16
160 Nm 0 Nm
0
10
20
30
40
50
60
70
80
R
M
S
D
(%
)
Torque in Bolt 1
1.77
15.86
160 Nm 0 Nm
0
10
20
30
40
50
60
70
80
R
M
S
D
(%
)
Torque in Bolt 4
1.33
19.41
Bolt 1
Loosened
Bolt 2
Loosened
Bolt 3
Loosened
Bolt 4
Loosened
T.C.Huynh, T.D.Nguyen, Q.N.Phạm / Tạp chí Khoa học và Công nghệ Đại học Duy Tân 02(39) (2020) 30-336 6
4. Conclusion
In this study, bolt looseness in a steel beam
connection was detected using a wireless
impedance monitoring method via the PZT
interface. Firstly, the wireless impedance
sensing system was adopted for automated and
cost-effective monitoring of impedance
responses from structural connections.
Secondly, the PZT interface technique was
employed to deal with the narrow measurable
frequency range of wireless impedance sensor
node and to predetermine sensitive frequency
ranges in the impedance measurement. Finally,
the wireless impedance sensor was integrated
with the PZT interface technique for damage
detection in a lab-scaled bolted beam
connection. Impedance responses of the test
structure were wirelessly measured under a set
of bolt-loosening events. The change in
impedance responses was quantified using the
RMSD index for distinguishing the bolt-
loosening events in the connection. The results
showed successful bolt-loosening detection and
thus demonstrated the feasibility of the wireless
impedance-based bolt-loosening detection
method. Despite the promising results, the
future study remains to test the practicality of
the method on large-scale steel structures in the
field.
Acknowledgement
This research is funded by Vietnam National
Foundation for Science and Technology
Development (NAFOSTED) under grant
number 107.01-2019.332
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